Fatigue Life Calculator

Fatigue Life Calculator – Estimate Cycles to Failure

Fatigue failure is responsible for the majority of mechanical component failures in engineering. This free fatigue life calculator helps engineers, designers, and students estimate how many stress cycles a component can withstand before failure using proven methods including the Basquin equation, S-N curve method, and Miner's Rule for cumulative damage analysis.

What Is Fatigue Life?

Fatigue life refers to the number of stress cycles a material or component can endure before failure occurs due to the initiation and propagation of cracks. Unlike static loading, cyclic loading — even below the yield strength — can cause progressive damage over time. Engineers use fatigue analysis to ensure components in bridges, aircraft, automotive parts, and industrial machinery operate safely within their design life.

Calculation Methods Explained

Basquin Equation (Stress-Life Method)

The Basquin equation relates the alternating stress amplitude to the number of cycles to failure:

σa = σ'f × (2N)^b
Rearranged: N = 0.5 × (σa / σ'f)^(1/b)

Where σa is the stress amplitude, σ'f is the fatigue strength coefficient, N is the cycles to failure, and b is the fatigue strength exponent (typically −0.05 to −0.15 for metals).

S-N Curve Method

The S-N (Stress-Number) curve is an empirical plot of stress amplitude versus cycles to failure for a given material. This calculator uses embedded material data for steel, aluminum, titanium, and copper to interpolate expected fatigue life at a given stress level.

Miner's Rule (Cumulative Damage)

Miner's Rule handles variable amplitude loading by accumulating damage from multiple stress blocks:

D = Σ(n_i / N_i)
Failure predicted when D ≥ 1

Each load block contributes a damage fraction n/N, where n is the applied cycles and N is the cycles to failure at that stress level. When the cumulative damage D reaches 1.0, fatigue failure is predicted.

Material Properties Used

Material	σ'f (MPa)	b	Endurance Limit (MPa)
Steel	900	−0.12	250
Aluminum	400	−0.11	140
Titanium	800	−0.10	350
Copper	300	−0.10	70

Practical Applications

Structural component design in aerospace and automotive engineering
Bridge and civil infrastructure life assessment
Rotating machinery shaft and gear fatigue analysis
Pressure vessel and pipeline inspection planning
Maintenance scheduling based on predicted service life
Product design validation for consumer goods under cyclic loading

Frequently Asked Questions

What is the difference between the Basquin equation and S-N curve method?

The Basquin equation is an analytical model parameterized by σ'f and b, while the S-N curve method uses empirical test data. In practice this calculator uses Basquin to generate the S-N curve, so both methods are mathematically equivalent when using the same material parameters.

What does the endurance limit mean?

The endurance limit (or fatigue limit) is the stress amplitude below which a material can theoretically sustain an infinite number of cycles without fatigue failure. For steel this is typically around 250 MPa; aluminum does not have a true endurance limit.

How does the safety factor affect the result?

The safety factor divides the calculated fatigue life, giving a conservative (lower) design life. A safety factor of 1.5 means the component is expected to last 1.5× longer than the minimum required life under the applied loading.